This invention relates generally to forming tubular parts in a lap joint. More specifically, this invention relates to a method of joining the tubes by a soldering/brazing method and hydroforming the assembly.
Automobiles, and other such motor vehicles, often include a frame and body assembly comprising several unique structures and/or shapes. In the past, vehicle body structures comprised panel and frame members, usually metallic such as steel. Steel has typically been used because of its relatively high strength, low cost and the ease by which it can be shaped into frame members or body panels. Recent studies have developed vehicular body structures that include relatively lighter materials, such as aluminum or magnesium, and/or irregularly shaped, thin-walled hydroformed structures that are designed to reduce the number of parts and the overall weight of the automobile.
The driving force for the introduction of hydroformed parts into the automotive industry is the desire to reduce the manufacturing cost and weight by consolidating parts. In a typical tubular space-frame application, hydroformed components are joined together to construct a tubular structure assembly. Another method for constructing a vehicle space-frame, discussed in U.S. Pat. No. 5,720,092 and assigned to the present assignee, involves hydroforming a pre-joined tubular structural assembly to achieve its final configuration. For both applications, a tube-to-tube lap joint is commonly used and critical to the success of such tubular space frames.
Traditional joining methods, prior to hydroforming, include fusion welding processes such as TIG, MIG, and laser welding. These processes may have drawbacks when applied to the tube-to-tube lap joint. For example, most arc welding processes involve a point heat source which normally requires more than one minute to travel around the tube to complete a circumferential joint. The time increases with larger diameter tubes. Laser welding is a faster mode, but is more expensive. Further most fusion welding processes, including laser welding, are sensitive to issues such as penetration, burn-through, clearance, and fit-up. The application of welding heat to relatively small areas of the thin-walled bulky structures often produces distortion of the parts at the weld region. This distortion can occur in the joining of steel parts but is particularly problematic in the joining of lower melting, lighter weight parts, such as those of aluminum or magnesium alloys. Moreover, these processes are sensitive to porosity caused by the presence of zinc coating on galvanized steel tubes. This may lead to potential leak paths during post-joining hydroforming of the tubes.
Thus, it is an object of the present invention to provide a new method for joining tubular components in a water-tight lap joint. It is a further object to employ such a method that utilizes lower temperatures and less heat so as to minimize distortion of metal in the region of the lap joint. Such a method would have particular utility in joining thin-walled tubular members, which can then be hydroformed.
This invention provides a joining process for securing two tubular members in a lap joint configuration, followed by a hydroforming process, which may include forming a supplementary mechanical lock in the lap joint.
In accordance with the invention, an insert end of a first tubular member is inserted within a receiver end of a second tubular member. A brazing/soldering process is employed to rapidly heat up the tubular joint area to elevated temperatures by induction/flame heating such that a metallic bond is formed between the tubes via the capillary action of the filler metal melted at the elevated temperature.
The brazing/soldering process provides a metallurgical bond between the overlapped tubes by wetting action at lower temperatures than the melting point of the tube base metal, which minimizes the risk of burn-through or lack of penetration. Certain brazing/soldering filler metals can alloy with galvanized coatings on the tubes and form the joint at temperatures much lower than zinc evaporation temperature; therefore zinc vapor porosity formed in many arc welding processes would not occur. Since heat is delivered uniformly across the joint area, the cycle time may be as short as 20 seconds and less heat distortion results than is usually encountered in welding thin-walled tubular parts in a lap joint.
Once a brazed/soldered joint is formed, the tubular assembly is then hydroformed into a final shape. During hydroforming, a supplementary mechanical lock may be created in the lap joint. The mechanical lock may take several forms such as an outward protrusion, an inward protrusion or a combination thereof. This provides additional integrity and strength to the lap joint.
Other objects and advantages of this invention will become apparent from a detailed description of specific embodiments that follow.